Can you check tailcap current with a digital multimeter with thick wire leads? Sounds like it's under-driven. The bigger reflector of the C8 should cause it to throw much better than a P60.
Here's a comparison beamshot I did here showing a KD C8 XM-L at 3A versus a Fake Solarforce L2 (P60) at 3A:
KD C8 (mouse out) and Fake L2 (Mouse over):
-Garry